Surface modified hexagonal boron nitride particles

ABSTRACT

Provided is a composition comprising hexagonal boron nitride particles having a surface and a substituted phenyl radical bonded to the surface, the substituted phenyl radical being represented by the structure: 
     
       
         
         
             
             
         
       
     
     where X is a radical selected from NH 2 —, HO—, R 2 OC(O)—, R 2 C(O)O—, HSO 3 —, NH 2 CO—, halogens, alkyl or aryl, including substituted aryl; where R 1  is hydrogen, alkyl or alkoxy, and R 2  is hydrogen, alkyl or aryl, including substituted aryl. A process for preparing the composition is also provided.

RELATED CASES

This application is related to co-owned applications [Attorney DocketNos. CL4658, CL4659 and CL4660], filed on the same day as the presentapplication.

FIELD OF THE INVENTION

The present invention deals with the preparation of surface modifiedhexagonal boron nitride particles that are suitable for incorporationinto polymers at high concentrations.

BACKGROUND

Meneghetti et al. U.S. 2007/0041918, discloses hexagonal boron nitridetreated with a zirconate and employed up to 75% by weight in polymers toprepare samples of improved thermal conductivity.

Sainsbury et al., WO 2008/140583, discloses amine (NH2) modified BNnanotubes by exposure to NH₃ plasma. Incorporation into a polymer ismentioned.

Ishida, U.S. Pat. No. 6,160,042, discloses boron nitride surface-treatedwith 1,4 phenylene diisocyanate incorporated into epoxy resins.

Mevellec et al., Chem. Mater. 2007, 19, 6323-6330, discloses that redoxactivation of aryl diazonium salts with iron powder in the presence ofvinylic monomers in aqueous solution leads to very homogeneous thinpolymer films strongly grafted on various surfaces such as (Au, Zn, Ti,stainless steel), glasses, carbon (nanotubes) or PTFE.

Epoxies and polyimides, are commonly used as components in printedcircuit boards. As the density of elements in electronic circuitsincreases, heat management is an ever increasing problem. Incorporationinto polymers of 50 volume % and more of hexagonal boron nitride (hBN)particles imparts enhanced thermal conductivity without compromisingelectrical insulation. In general, surface treatment of the BN particlesis required in order to achieve adequate dispersion and moldability.

However, even with surface treatment, the high loading of hBN particlesrequired for improved thermal conductivity in a given polymer causes alarge increase in viscosity with concomitant degradation inprocessibility. This is especially a problem in producing substratefilms for flexible printed circuit boards.

SUMMARY OF THE INVENTION

The invention provides a composition comprising hexagonal boron nitrideparticles having a surface and a substituted phenyl radical bonded tothe surface, the substituted phenyl radical being represented by thestructure:

where X is a radical selected from NH₂—, HO—, R²OC(O)—, R²C(O)O—, HSO₃—,NH₂CO—, halogens, alkyl and aryl, including substituted aryl; R¹ ishydrogen, alkyl or alkoxy, and R² is hydrogen, alkyl or aryl, includingsubstituted aryl.

Further provided in the present invention is a process comprisingreacting particles of hexagonal boron nitride with a substituted phenyldiazonium chloride in an alcohol/water solution in the presence ofmetallic iron and HCl, and recovering the reaction product therefrom;wherein the alcohol/water solution has a water concentration of at least50% by volume; and wherein the substituted phenyl diazonium chloride isrepresented by the formula

where X is selected from NH2-, HO—, R²OC(O)—, HSO3-, NH2CO—, halogens,alkyl and aryl, including substituted aryl; R¹ is alkyl or alkoxy, andR² is hydrogen, alkyl or aryl, including substituted aryl.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic representation of the structure of a hexagonalboron nitride particle.

FIG. 2 a is a transmission electron micrograph (TEM) of the plateletedges of hBN as received (Comparative Example A).

FIG. 2 b is a TEM of the platelet edges of the SMhBN prepared accordingto Example 1.

FIG. 3 a. is a TEM of the basal plane in the SMhBN of Example 1, a viewfrom the platelet edges.

FIG. 3 b is a TEM of the platelet edges in the SMhBN of Example 1, aview from the basal plane.

DETAILED DESCRIPTION

In the present invention a novel treatment for hBN is provided thatproduces a new chemical species on the surface of hBN particles, makingthem highly compatible with thermo-set polymers such as epoxies andpolyimides, and provides tough, flexible substrates for flexible printedcircuit boards.

Whenever a range of values is provided herein, the range is intended toencompass the endpoints of the range unless it is specifically stated tobe otherwise.

In one embodiment, the polymer is a thermoset polymer, or the uncured(or uncross-linked) thermosettable polymeric precursor correspondingthereto. When the precursor polymer is subject to elevated temperatureit undergoes a cross-linking reaction (or curing reaction) orimidization that converts a flowable and/or formable polymer into anon-flowable, non-formable polymer. Suitable thermoset polymers includebut are not limited to polyimides and cured epoxy resins. Suitablethermosettable precursor polymers include polyamic acids and epoxidepolymers. The curing process may or may not involve the addition of across-linking agent. Conversion of some polymers requires addition of across-linking agent.

The term “flowable” refers to a viscous mass which is displaced upon theapplication of a shear force. The term “formable” refers to a viscousmass which can be formed into a shaped article and will hold the shapefor sufficient time that it can be set in that shape, either by cooling,curing, or imidization. In general, all formable masses are flowable,but not all flowable masses are formable. Formable masses are generallyof higher viscosity than non-formable but flowable masses.

Polyimides are uncrosslinked thermoplastic polymers which generallydecompose before they melt. Polyimides do not exhibit flow attemperatures as high as 500° C. However, as used herein, the term“thermoset polymer” encompasses polyimides, and the term “thermosettablepolymer” encompasses polyamic acids.

The present invention provides films having a thickness less than 500μm. Films are generally prepared, for example, by melt casting orsolution casting onto a release surface. In one embodiment, the filmshave a thickness in the range of 10 to 100 μm. In a further embodiment,the films have a thickness in the range of 15 to 80 μm. Films that aretoo thin may exhibit insufficient toughness for use as substrates forprinted circuit boards. Films that are too thick may exhibitinsufficient flexibility to be useful as substrates for flexible printedcircuit boards.

Viscosity as described herein is determined at room temperatureaccording to ASTM D 2196-05, using a Brookefield viscometer, ModelDV-II+, spindle #28.

In one aspect, the present invention provides a composition (hereindesignated SMhBN) comprising hexagonal boron nitride particles having asurface and a substituted phenyl radical bonded to the surface, thesubstituted phenyl radical being represented by the structure:

where X is a radical selected from NH₂—, HO—, R²OC(O)—, R²C(O)O—, HSO₃—,NH₂CO—, halogens, alkyl and aryl, including substituted aryl; where R¹is hydrogen, alkyl or alkoxy, and R² is hydrogen, alkyl or aryl,including substituted aryl.

In one embodiment, R¹ is hydrogen. In another embodiment, X is NH₂— orHO—. In another embodiment, X is NH₂—. In another embodiment, X is HO—.The substituted phenyl radical is bonded to the surface of the hBN.

Hexagonal boron nitride particles are known to be represented by thestructure shown in FIG. 1, consisting of multiple stacked, registeredplatelets, offering a high surface area for reaction. The white andblack alternating circles represent nitrogen and boron atoms arranged inhexagonal lattices. There are no particular size limitations for the hBNparticles employed herein. Typical commercially available hBN particlesfall in size range of about 0.7 micrometer, about 10 to 12 μm, and about14 to 15 μm. As the particle size grows smaller, the particle is harderto disperse. On the other hand, as the particle size grows larger, thecomposite film may exhibit undesirable surface roughness.

In one embodiment, the SMhBN has a concentration of the substitutedphenyl radical in the range 0.1 to 4.0% by weight on the basis of SMhBN.

In another aspect the invention provides a process for producing theSMhBN, comprising reacting particles of hexagonal boron nitride with asubstituted phenyl diazonium chloride in an alcohol/water solution inthe presence of metallic iron and dilute HCl, and recovering thereaction product therefrom; wherein the alcohol/water solution has awater concentration of at least 50% by volume; and wherein thesubstituted phenyl diazonium chloride is represented by the formula

where X is selected from NH₂—, HO—, R²OC(O)—, HSO₃—, NH₂CO—, halogens,alkyl and aryl, including substituted aryl; R¹ is alkyl or alkoxy, andR² is hydrogen, alkyl or aryl, including substituted aryl. In oneembodiment, R¹ is hydrogen. In another embodiment, X is NH₂— or HO—. Inanother embodiment, X is NH₂—. In another embodiment, X is HO—.

In one embodiment, the water/alcohol solution is at least 80% by volumewater.

Suitable substituted phenyl diazonium chlorides may be prepared usingknown methods. For example, preparation of 4-amino-benzene diazoniumchloride is described in United Kingdom Patent GB1536320. preparation of4-hydroxy-benzene diazonium chloride is described in Grieve et al.,Helvetica Chimica Acta; English; 68; 1985; 1427-1443, and preparation of4-carboxy-benzene diazonium chloride is described in Weedon; AmericanChemical Journal; 33; 1905; 417.

In one embodiment, the molar ratio of the substituted benzene diazoniumchloride to hBN in the reaction mixture is in the range of 0.005:1 to0.1 to 1. In one embodiment, iron is added in molar excess over theamount of substituted phenyl diazonium chloride. In a furtherembodiment, iron is added as a powder. In a still further embodiment,the iron powder is less than-1-2 mm in at least one dimension (10 mesh).

The amount of modifier residing on the surface of the hBN depends uponthe surface area of the hBN and the reactivity of the phenyl radical. Aphenyl radical containing an electron donating group (such as amine oralkyl) is more reactive, while a phenyl radical containing an electronwithdrawing group (such as COOH or OH) is less reactive. The effect ofhBN surface area on the amount of uptake of a given phenyl radical isshown in Table 1:

TABLE 1 surface Particle size area Phenyl radical (hBN) (m2/g)concentration NX1 (0.7 um) 20 1.93% PT120 (12-13 um) 2.0 0.42%

It has been found that the reaction rate is governed by the reactivityand half life of the radical, and the susceptibility of the substrate toundergo reaction. The lower reactivity hydroxyphenyl radical results asmaller amount of substituted phenyl radical surface modification of hBNthan does the higher reactivity aminophenyl radical when the startingmaterials are in the same ratio.

In one embodiment of the process, iron is present in excess. The iron isthought to provide a surface upon which the reduction of the diazoniumsalt to the radical takes place The reduction reaction may proceed to ahigher degree if iron is present in excess. An excess of iron insuresthat all the diazonium salt is reduced, avoiding the explosion hazardassociated with having residual diazonium salt.

Dilute acid reduces the rate of reaction of the diazonium salt andthereby reduces explosion hazard. It has been found that for theprocesses disclosed herein dilute HCl (in one embodiment, 0.1-1.0 M, ina further embodiment 0.3 to 0.7 M) is satisfactory. Because of therelative stability of the diazonium chloride, use of HCl is desirable toavoid possible explosion.

In one embodiment the reaction is conducted at room temperature.

In one embodiment of a batch process, an aqueous solution of substitutedbenzene diazonium chloride is mixed with a dispersion of hBN inwater/alcohol mixture. The iron particles are then added to the mixtureand stirred for several minutes, followed by addition of HCl, which isfollowed by further stirring for about 30 minutes. The concentration ofHCl in the reaction mixture does not exceed 0.1 M.

In one embodiment, the iron particles are removed with a magnet, and theSMhBN is isolated by filtration and drying.

Viscosity suitable for handling and mixing the reaction mixture isobtained by combining alcohol with water. Suitable alcohols include, butare not limited to, the C₁ to C₆ alkyl alcohols, including methanol,ethanol, and propanol. Concentrations of alcohol greater than 50% byvolume have little additional effect on viscosity, but represent asafety hazard. The diazonium chloride compositions employed herein areinherently unstable, particularly when not in solution. Should adestabilizing event occur, the presence of a flammable liquid such as analcohol would add undesirable fuel to the fire.

In one embodiment a 50/50 mixture by volume of water and alcohol isemployed as the solvent for the for the surface modification reaction.In a further embodiment, a mixture of 80 vol-% water and 20 vol-%methanol is employed.

In another embodiment, the SMhBN is combined with a polymer to prepare apolymer composition comprising a polymer and a plurality ofsurface-modified hexagonal boron nitride particles dispersedtherewithin, the surface modified hexagonal boron nitride particlecomprising a hexagonal boron nitride particle having a surface and asubstituted phenyl radical bonded to the surface, the substituted phenylradical being represented by the structure:

where X is a radical selected from NH₂—, HO—, R²OC(O)—, R²C(O)O—, HSO₃—,NH₂CO—, halogens, alkyl and aryl, including substituted aryl; R¹ isalkyl or alkoxy, and R² is hydrogen, alkyl or aryl, includingsubstituted aryl.

In one embodiment, R¹ is hydrogen. In another embodiment, X is NH₂— orHO—. In another embodiment, X is NH₂—. In another embodiment, X is HO—.

In one embodiment, the polymer comprises polyamic acid. In a furtherembodiment, the polyamic acid is in solution. In another embodiment, thepolymer is a polyimide. In another embodiment, the polymer is anepoxy-containing polymer either in the liquid state or in solution. Inanother embodiment, the polymer is a cured epoxy resin.

Polyimide chemistry is very well-known in the art; see for example thearticle in the Encyclopedia of Polymer Science and Technology by Bryant,DOI 10.1002/0471440264.pst272.pub2. Condensation polyimides are normallyprepared by imidization of a corresponding polyamic acid. Suitablepolyamic acid compositions include but are not limited to the polymersprepared by the reaction of equimolar amounts of diamines andtetracarboxylic dianhydrides (or the dianhydride's acid ester or theacid halide ester derivative) in suitable solvents. In one embodiment,the anhydride moiety is selected from the group consisting ofpyromellitic dianhydride (PMDA), 4,4-oxydiphthalic dianhydride (ODPA),3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA),3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA),2,2′-bis(3,4-dicarboxyphenyl) 1,1,1,3,3,3,-hexafluoropropane dianhydride(FDA), and 2,3,6,7-naphthalene tetracarboxylic dianhydrides.

Suitable diamines include 1,4-phenylene diamine (PPD), 1,3-phenylenediamine (MPD), 4,4′-diaminodiphenyl ether (4,4′-ODA),3,4′-diaminodiphenyl ether (3,4′-ODA), 1,3-bis-(4-aminophenoxy)benzene(APB-134), 1,3-bis-(3-aminophenoxy)benzene (APB-133),2,2′-bis-(4-aminophenyl)-1,1,1,3,3,3-hexafluoropropane (6F diamine), andbis[4-(4-aminophenoxy)phenyl]ether (BAPE).

The polyamic acid formed may be either a homopolymer, or randomcopolymer if more than one diamine and/or dianhydrides are used for thepolymerization. Segmented copolymers can be formed by polymerizinginitially with an excess of a first diamine or first dianhydrides andthen adding different dianhydrides or diamines respectively.

Polyamic acids, by virtue of their solubility in a variety of solvents,are more highly processible than the corresponding polyimide. It iscommon practice in the art to perform any mixing and forming operationson the polyamic acid composition followed by imidization. Polyimides arewell-known to be highly inert to solvents and elevated temperatures.

Epoxy chemistry is very well-known in the art; see for example thearticle in the Encyclopedia of Polymer Science and Technology by Phamand Marks, DOI 10.1002/0471440264.pst119. “Epoxy resins” is the term ofart employed to refer to a cured epoxy. An uncured epoxy has a cyclicepoxide group along the polymer chain. A cured epoxy is one in which apreponderance of the cyclic epoxide groups have undergone reaction witha curing agent (also known as a cross-linking agent) to form cross-linksamong polymer chains thereby forming a rigid, substantially inert 3-Dnetwork of polymer chains. Suitable uncured epoxy compositions compriseone or more multifunctional or difunctional epoxy resins, an epoxycuring agent, a toughener and a curing accelerator.

Suitable multifunctional epoxy resins include but are not limited tophenol-novolac epoxy, cresol-novolac epoxy, tetraglycidyl ether ofdiaminodiphenylmethane, triglycidyl tris(hydroxylphenyl)methane,trigylcidyl ether of p-aminophenol, naphthalene epoxy resin, thetrigylcidyl derivative of cyanuric acid, the epoxy derivative ofbiphenol. Suitable difunctional epoxy resins include but are not limitedto glycidyl ethers of bisphenol A, bisphenol F and bisphenol S, andreactive diluants such as aliphatic epoxies.

Suitable curing agents for epoxies include but are not limited toamines, amides, anhydrides, polyamides, polyamine adducts, organicacids, phenols and phenolic resins. Phenolic curing agents areparticularly preferred for their moderating effects on viscosity of thecomposition and moisture absorption, good electrical and hightemperature mechanical properties. Suitable phenolic curing agentsinclude bisphenol A, xylok type phenol resin, dicyclopentadiene type ofphenol, terpene modified phenolic reisn, phenolic resin andpolyvinylphenol.

In one embodiment, the epoxy composition also includes polymerictoughening agents having average molecular weight in the range5000-100000, and being soluble in the the epoxy resin solution. Suitablepolymeric tougheners include but are not limited to phenoxys, acrylics,polyamides, polycyanates, polyesters and polyphenylene ethers.

In a further embodiment, the epoxy composition also contains a curingaccelerator. Suitable curing accelerators include but are not limited toamines, guanidine, imidazoles, triphenyl phosphine, triphenylphosphonium tetrafluoroborate, or epoxy adducts thereof.

Uncured epoxies, by virtue of their inherent liquidity at roomtemperature or solubility in a variety of solvents are more highlyprocessible than the corresponding cured epoxy. It is common practice inthe art to perform any mixing and forming operations on the uncuredepoxy composition followed by curing. Cured epoxy polymers arewell-known to be highly inert to solvents and elevated temperatures.

In one embodiment, the polymer composition comprises 30-70% by weight ofthe SMhBN. In a further embodiment, the polymer composition comprises40-65% by weight of the SMhBN. In one embodiment, the average equivalentspherical diameter of the SMhBN is in the range of 0.5 μm to 50 μm. In afurther embodiment, the average equivalent spherical diameter of theSMhBN is in the range of 0.5 to 25 μm.

It is not necessary to break up agglomerated SMhBN. Usually the SMhBNdoes not agglomerate readily. PT620 is sold as agglomerated hBN by thevendor. In some embodiments, agglomerated SMhBN is preferred forimproved thermal conductivity. Agglomerates of SMhBN can be soft orhard. Hard agglomerates form by surface modifying PT620 Agglomerateshaving sizes in the range 0.5-50 micrometer are suitable. In oneembodiment, the SMhBN dispersed in the polymer can be characterized by aplurality of particle size distribution peaks characteristic of aso-called multi-modal particle size distribution.

Particles having a size of about 1/10 of the desired film thickness orless are desirable to make smooth-surfaced films having isotropicallybalanced mechanical and thermal properties. Increasing particle sizeintroduces anisotropy and surface roughness.

In another aspect, the invention provides a process comprising combininga plurality of surface-modified hexagonal boron nitride particles with apolymer solution comprising a solvent, and, extracting the solvent,wherein the surface-modified hexagonal boron nitride particles comprisehexagonal boron nitride particles having a surface and a substitutedphenyl radical bonded to the surface, the substituted phenyl radicalbeing represented by the structure:

where X is a radical selected from NH₂—, HO—, R²OC(O)—, R²C(O)O—, HSO₃—,NH₂CO—, halogens, alkyl and aryl, including substituted aryl; R¹ isalkyl or alkoxy, and R² is hydrogen, alkyl or aryl, includingsubstituted aryl.

In one embodiment, R¹ is hydrogen. In another embodiment, X is NH₂— orHO—. In another embodiment, X is NH₂—. In another embodiment, X is HO—.

In one embodiment, the process further comprises combining a dispersionof SMhBN in a first organic liquid, with a solution of a polymer in asecond organic liquid, with the proviso that the first and secondorganic liquids are miscible, and are both solvents for the polymer.

When the polymer is a polyamic acid, suitable organic liquids includebut are not limited to N,N′-dimethyl acetamide, N,N′-dimethyl formamide,N-methyl pyrrolidone, tetramethyl urea, dimethyl sulfoxide, andhexamethyl phosphoramid.

When the polymer is an epoxy resin, suitable organic liquids include butare not limited to acetone, methyl ethyl ketone, cyclohexanone,propylene glycol mono methyl ether acetate, carbitol, butyl carbitol,toluene, and xylene.

In one embodiment, the SMhBN is dispersed within the first organicliquid at a solids content in the range of 20 to 70% by weight. In afurther embodiment, solids content is in the range of 30-40% by weightThe dispersion is readily achieved by simple mixing using any mechanicalstirrer.

In one embodiment, the first and second organic liquids are bothdimethylacetamide (DMAc), and the concentration of SMhBN is 30-40%. Inthis embodiment, the dispersion viscosity is below 100 cp at roomtemperature. It has been found that viscosity below 100 cp is ofteninsufficient to obtain homogeneous mixing of the dispersed SMhBN in thepolymer solution. In a further embodiment, a masterbatch of 30-40% SMhBNin DMAc is prepared by adding about 5-20% by weight of polymer basedupon the weight of the SMhBN. The presence of the polymer increases theviscosity above 100 cp, and allows good mutual dispersion of polymer andSMhBN, resulting in a masterbatch comprising a relative concentration ofSMhBN/polymer in the range of 96/4 to 80/20. The masterbatch so producedcan then be mixed with further polymer solution to produce the finaldesired concentration of SMhBN in the polymer, namely SMhBN in the rangeof 30-70% by weight on the basis of total solids, in one embodiment,40-65% by weight of the total solids. In one embodiment, the polymer ispolyamic acid. In an alternative embodiment, the polymer is uncuredepoxy resin.

The polymeric dispersions are produced by high shear mixing. Suitablehigh shear mixers include but are not limited to homogenizers (availablefrom Silverson Mechanics Inc, East Long Meadow, Mass.), blenders,ultrasonic agitators, or roller mills with grinding media. Suitableranges for the viscosity of the dispersion, comprising organic liquid,SMhBN and polymer, for high shear mixing can be in the range of 100 to2000 cp, preferably in a viscosity range of 200 to 1500 cp.

In one embodiment, the second organic liquid can be a liquid polymer,for example, a liquid epoxy resin having relatively low molecular weightand viscosity Liquid epoxy resins suitable for use in the process aremade from bisphenol A, bisphenol F, epoxy modified liquid rubber, epoxyresins derived from polyhydric alcohols such as ethylene glycol,propylene glycol, neopentyl glycol and the like. Reactive diluents suchas allylglycidyl ether, glycidyl methacrylate, and allylphenyl glycidylether can also be used to improve dispersion of the SMhBN. It has beenfound that the viscosity suitable for high shear mixture is in the range200 to 1500 cp. Solvents can be used to dilute the dispersion until itis within the above viscosity range.

In another embodiment, the second organic liquid is selected fromN,N′-dimethyl acetamide, N,N′-dimethylformamide or N-methyl pyrrolidoneand the polymer is dissolved therewithin at a concentration in the rangeof 10 to 30% by weight, preferably 15 to 25% by weight. When the secondorganic liquid is a liquid polymer, to achieve miscibility with thefirst organic liquid, the liquid polymer is dissolved in the firstorganic liquid when they are combined. In one embodiment, the firstorganic liquid and the second organic liquid are the same. In oneembodiment, the first and second organic liquids are dimethyl acetamide(DMAc).

In one embodiment a dianhydride corresponding to the dianhydride moietyin the polyamic acid SMhBN/polyamic acid dispersions, increasing themolecular weight of the imidized polymer may be desired to improve themechanical properties of the film. A suitable method is to add thecorresponding dianhydride (part of polyamic acid) in 10 to 25 milligramquantities, stir the dispersion for ten minutes until the anhydridecompletely dissolved in the dispersion and measure the viscosity of thedispersion. The addition of the anhydrides in small quantities, mixingthe dispersion and measuring the viscosity of the dispersion shall becontinued until a dispersion having viscosity in the range 60,000 to150,000 cp and preferably a viscosity in the range 75,000 to 100,000 cpis achieved. These viscosities are generally associated with polyamicacid solutions that are suitable for solution casting of films followedby imidization to tough, strong polyimide films.

In the case of SMhBN/epoxy dispersions, it is preferable to have a totalsolid content in the range 40 to 80% by weight, preferably in the range50 to 70% by weight. Also, it is preferable to have the viscosity of thedispersion in the range 100 to 2000 cp, preferably in the range 200 to1500 cp.

The composite can be shaped into an article such as a film or sheet, arod or other stock shape, followed by curing, imidization or othermethod for effecting the transition from a flowable or formablecomposite to a non-flowable, non-formable composite. Depending upon thespecific features and components of the uncured composite, it may bedesirable to extract at least a portion of the organic liquid in orderto get a suitably formable composite. Extraction may be effected by anyconvenient method including but not limited to, heating in a vacuum ovenor air circulating oven, or evaporation by casting on a heated drum.

In one embodiment, the second organic liquid is a liquid polymer, andonly the first organic liquid is subject to extraction. In oneembodiment the second organic liquid is a solvent for a polymersolution, and is also subject to extraction.

In another aspect, the invention provides a film having a thickness ofless than 500 μm comprising a polymer and a plurality ofsurface-modified hexagonal boron nitride particles dispersedtherewithin, wherein the surface modified boron nitride particlecomprises a hexagonal boron nitride particle having a surface and asubstituted phenyl radical bonded to the surface, the substituted phenylradical being represented by the structure:

where X is a radical selected from NH₂—, HO—, R²OC(O)—, R²C(O)O—, HSO₃—,NH₂CO—, halogens, alkyl or aryl, including substituted aryl; where R¹ isalkyl or alkoxy, and R² is hydrogen, alkyl or aryl, includingsubstituted aryl.

In one embodiment, R¹ is hydrogen. In another embodiment, X is NH₂— orHO—. In another embodiment, X is NH₂—. In another embodiment, X is HO—.

In one embodiment, the polymer comprises polyamic acid. In a furtherembodiment, the polyamic acid is in solution. In another embodiment, thepolymer is a polyimide. In another embodiment, the polymer is anepoxy-containing polymer either in the liquid state or in solution. Inanother embodiment, the polymer is a cured epoxy resin.

In one embodiment, the polymer composition comprises 30-70% by weight ofthe SMhBN. In a further embodiment, the polymer composition comprises40-65% by weight of the SMhBN. In one embodiment, the average equivalentspherical diameter of the SMhBN in the polymer is in the range of 0.5 μmto 50 μm. In a further embodiment, the average equivalent sphericaldiameter of the SMhBN in the polymer is in the range of 0.5 to 25 μm. Inone embodiment, the SMhBN dispered in the polymer can be characterizedby a plurality of particle size distribution peaks characteristic of aso-called multi-modal particle size distribution.

In one embodiment, the film further comprises an organic liquid. In oneembodiment the film is formable. In an alternative embodiment the filmis non-formable. In general, the formable film is a precursor to thenon-formable film. In one embodiment, the formable film is first formedand then, in a single continuous process, converted into thenon-formable film. In an alternative embodiment, the formable film isprepared as, for example, roll stock. The roll stock is transferred to afabricator who forms the formable film into a complex shape, and thencauses it to be cured or imidized to the non-formable state whileholding the complex shape.

In a further aspect, the present invention provides a process comprisingcasting onto a surface a dispersion of a plurality of surface modifiedhexagonal boron nitride particles in a solution of a polymer in asolvent, forming the thus cast dispersion into a viscous liquid film,and, extracting the solvent to form a film having a thickness of lessthan 500 μm; wherein the surface modified hexagonal boron nitrideparticles comprise hexagonal boron nitride particles having a surfaceand a substituted phenyl radical bonded to the surface, the substitutedphenyl radical being represented by the structure:

where X is a radical selected from NH₂—, HO—, R²OC(O)—, R²C(O)O—, HSO₃—,NH₂CO—, halogens, alkyl and aryl, including substituted aryl; R¹ isalkyl or alkoxy, and R² is hydrogen, alkyl or aryl, includingsubstituted aryl.

In one embodiment, R¹ is hydrogen. In another embodiment, X is NH₂— orHO—. In another embodiment, X is NH₂—. In another embodiment, X is HO—.

The term “casting” refers to the process by which a polymericcomposition is applied to a surface to form a film. Suitable castingprocesses include but are not limited to adjustable micrometer filmapplicator (doctor blade), wire wound metering rod (Meyer rod) or slotdie, which is commonly used in large scale production.

A composition suitable for film casting (“casting composition”)comprises an organic liquid, a polymer dissolved therewithin, and aplurality of SMhBN particles dispersed therewithin. In one embodiment,the casting composition comprises a polymer composite comprising amixture of a first organic liquid, a second organic liquid miscible inthe first organic liquid, a polymer dissolved in at least the first orthe second organic liquid, and a plurality of SMhBN particles dispersedtherewithin. In one embodiment the polymer is a polyamic acid. In analternative embodiment, the polymer is an uncured epoxide polymer.

In a further embodiment, the first and second organic liquids are thesame. In a still further embodiment, the first and second organicliquids are DMAc. In an alternative embodiment, the casting compositioncomprises a polymer composite comprising a mixture of a liquid polymerand a first organic liquid miscible in the liquid polymer, and aplurality of SMhBN particles dispersed therewithin. In a furtherembodiment, the first organic liquid is DMAc. In a still furtherembodiment, the liquid polymer is a liquid epoxide polymer.

In one embodiment, the surface upon which the casting composition iscast is selected to provide adhering contact with the cast film aftersolvent extraction and curing or imidizing, thereby resulting in amulti-layer laminate of which at least one layer comprises the cured orimidized film. In one embodiment, the casting composition comprises apolyamic acid and a suitable surface is a polyimide. In a furtherembodiment, the casting composition comprises an epoxy resin and asuitable surface is a cured epoxy. In another embodiment the surface isa that of a metal foil. In a further embodiment, the metal foil iscopper foil.

When casting the film on a metal foil, the metal surface can beroughened to achieve adhesion. In one embodiment, casting is effectedonto the matte side of an electrodeposited copper foil. In analternative embodiment, casting is effected onto the shiny surface of anelectrodeposited copper foil, and the foil acts as a release layer. Ifcasting is effected onto the matte surface but the polymer is not fullycured or imidized, it may still be possible to separate the two layersby ordinary methods. However, if the polymer is fully cured or imidized,the layers are tightly bound.

Other materials suitable for use as release layers include but are notlimited to polyethylene, polyvinyl chloride, polyethylene terephthalate,polyethylene naphthalate, and polycarbonate.

The quality of the films so-cast depends on the uniformity of thedispersion in the coating composition, absence of trapped air bubbles,the viscosity of the dispersion, the accuracy in metering of the castingcomposition to produce uniform thickness, etc. The uniformity of thecasting composition depends in part on achieving wetting of theparticles by the solvent and the resin, in part on particle size andagglomeration. It is preferable to disperse the particules using a highshear mixture in a moderately viscous resin solution. The particles tendto settle during storage of a dispersion in a low viscosity polymersolution. A suitable epoxy coating composition has a viscosity in therange 100-2000 cp. A suitable polyamic acid coating composition has aviscosity in the range of 75000-1 00000 cp. Air bubbles can be removedby stirring under vacuum.

In a further aspect, the present invention provides a process comprisingdisposing an electrically conductive metallic layer upon the surface ofa polymer composite film less than 500 μm in thickness, having asurface, followed by the application of pressure or a combination ofpressure and heat to effect bonding therebetween, the polymer compositefilm comprising a polymer and a plurality of surface-modified hexagonalboron nitride particles dispersed therewithin comprising hexagonal boronnitride particles having a surface and a substituted phenyl radicalbonded to the surface, the substituted phenyl radical being representedby the structure:

where X is a radical selected from NH₂—, HO—, R₂OC(O)—, R₂C(O)O—, HSO₃—,NH₂CO—, halogens, alkyl or aryl, including substituted aryl; where R₁ isalkyl or alkoxy, and R₂ is hydrogen, alkyl or aryl, includingsubstituted aryl.

In one embodiment, R₁ is hydrogen. In another embodiment, X is NH₂— orHO—. In another embodiment, X is NH₂—. In another embodiment, X is HO—.

In one embodiment the metal is copper. In a further embodiment thecopper is in the form of a copper foil. In one embodiment, theconductive metal layer is in the form of conductive pathways.

In one embodiment the process further comprises disposing an adhesivelayer between the polymeric composite film and the conductive metallayer.

Other suitable conductive metal layers suitable include, but are notlimited to stainless steel, copper alloys, aluminum, gold, silver,tungsten, nickel, and alloys thereof.

Suitable materials for use in the adhesive layer include but are notlimited to epoxies, acrylics, phenolic, thermoplastic polyimides,poly-etherimides, polyester, polyamide, polyamide-imides, polyimide,polyetherimides, polyether-ketones, polyether-sulfones and liquidcrystal polymers.

In a further aspect, the invention provides a multi-layer articlecomprising a layer of an electrically conductive metal adheringlycontacting the surface of a polymeric composite film less than 500 μm inthickness, having a surface, the polymeric composite film comprising apolymer and a plurality of surface-modified hexagonal boron nitrideparticles dispersed therewithin comprising hexagonal boron nitrideparticles having a surface and a substituted phenyl radical bonded tothe surface, the substituted phenyl radical being represented by thestructure:

where X is a radical selected from NH₂—, HO—, R²OC(O)—, R²C(O)O—, HSO₃—,NH₂CO—, halogens, alkyl and aryl, including substituted aryl; R₁ isalkyl or alkoxy, and R² is hydrogen, alkyl or aryl, includingsubstituted aryl.

In one embodiment, R¹ is hydrogen. In another embodiment, X is NH₂— orHO—. In another embodiment, X is NH₂—. In another embodiment, X is HO—.

In one embodiment the metal is copper. In a further embodiment thecopper is in the form of a copper foil. In one embodiment, theconductive metal layer is in the form of conductive pathways. In oneembodiment the multi-layer article further comprises an adhesive layerbetween the polymeric composite film and the conductive metal layer.

The metal layers can be formed of any metal, including copper, gold,silver, tungsten or aluminum. In one embodiment, the metal layer is acopper foil. The copper foil can be created in any manner known in theart including electro deposition or rolled copper foil.

In one embodiment, the adhesive layer comprises a thermoplastic polymer.Suitable thermoplastic polymers include but are not limited topolyimides made by reacting aromatic di-anhydrides with aliphaticdiamines. Other materials useful as a dielectric adhesive layer includepolyester, polyamide, polyamide-imides, polyetherimides,polyether-ketones, polyether-sulfones and liquid crystal polymers. In analternative embodiment, the adhesive layer comprises a thermosetpolymer. Suitable thermoset polymers include but are not limited toepoxies, phenolic resins, melamine resins, acrylic resins, cyanateresins, and combinations thereof. Generally, the adhesive layer has athickness in the range of 3 to 35 μm, and has an in-plane coefficient ofthermal expansion (CTE) of 25 to 90 ppm/° C. at 20° C.

In one embodiment, the adhesive layer comprises a polyimide with glasstransition temperatures of 150 to 350° C. Generally, bondingtemperatures are 20 to 50 degrees higher than the glass transitiontemperature. In a further embodiment, the adhesive polyimide issynthesized by condensing an aromatic dianhydride with a diaminecomponent comprising 50 to 90 mol-% aliphatic diamine and 1 to 50 mol-%aromatic diamine. In a further embodiment, the aliphatic diamine ishexamethylene diamine (HMD) and the aromatic diamine is1,3-bis-(4-aminophenoxy)benzene, the aromatic dianhydride is acombination of 3,3′,4,4′-benzophenone tetracarboxylicdianhydride (BTDA)and 3,3′,4,4′-biphenyl tetracarboxylic polymer having a glass transitiontemperature in the range of 150 to 200° C.

In an another embodiment, the adhesive is a heat-sealable copolyimidecomprising from 60 to 98 mole % of repeating imide units of the formulaI

and from 2 to 40 mole % of other repeating imide units of the formula II

wherein R is the radical of a tetravalent organic carboxylic dianhydrideselected from the group consisting of pyromellitic dianhydride,4,4′-oxydiphthalicdianhydride, 3,3′,4,4′-benzophenone tetra-carboxylicdianhydride, 2,2′,3,3′-benzophenone tetracarboxylicdianhydride,3,3′,4,4′-biphenyl tetracarboxylicdianhydride, 2,2′,3,3′-biphenyltetracarboxylicdianhydride,2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride,bis(3,4-dicarboxyphenyl)sulfone dianhydride and m-phenylenebis(trimellitate)dianhydride; and wherein R′ is the radical of adivalent aromatic or aliphatic diamine selected from the groupconsisting of p-phenylenediamine, hexamethylene diamine, heptamethylenediamine, octamethylene diamine, 4,4′diaminodiphenylether,3,4′-diaminodiphenylether, 1,3-bis(4˜aminophenoxy)benzene,1,2-bis(4aminophenoxy)benzene, 1,3-bis(4-aminobenzoyloxy)benzene,4,4′-diaminobenzanilide, 4,4′-bis(4-aminophenoxy)phenyl ether and apolysiloxanediamine, provided that the repeating imide units of formula(I) are different from the repeating imide units of formula (II).

The conductive metal layer thickness can generally be in the range of 2to 500 μm, and in one embodiment, the conductive metal layer thicknessis in the range of 3 to 35 μm. In one embodiment, the conductive metallayer is a copper foil having a CTE in a range between 15 and 17ppm/° C.at 20° C.

The conductive layer can be pre-treated mechanically or chemically toimprove laminate adhesion. Pretreatments commonly practiced in the artinclude but are not limited to electro-deposition; immersion-depositionalong the bonding surface of a thin layer of copper, zinc, chrome, tin,nickel, cobalt, other metals, and alloys of these metals. Apart fromroughening the surface, chemical pretreatment may also lead to theformation of metal oxide groups, enabling improved adhesion between themetal layer and dielectric multilayer. In one embodiment, thepretreatment is applied to both sides of the metal, enabling enhancedadhesion for both sides of the metal.

In one embodiment, a resin-coated foil is prepared by coating ametallic, preferably copper, foil with a casting composition describedsupra. In one embodiment, the casting composition is metered onto amoving copper foil using a combination of coating and compression rollsin a continuous process. Other suitable coating processes are blade orknife coating, slot or extrusion coating, gravure coating, slidecoating, and curtain coating. In one embodiment, the coated foil isdried, typically in an oven, to increase the viscosity of the uncuredcoating, partially curing or imidizing it to form a so-called B-stagecomposition. In one embodiment, the process is a continuous coatingprocess, and the foil coated with the B-stage composition can be woundon a roll for further use. In some embodiments, a first coating layer isfully cured, and then the coated foil is further coated with one or moreadditional layers.

In one embodiment, the conductive metal layer is formed on the surfaceof the film, by dry plating or wet plating. Dry plating methods known inthe art include sputtering or ion plating. In wet plating, the surfaceof the cured layer is first roughened with an oxidizing agent such as apermanganate, a bichromate, ozone, hydrogen peroxide/sulfuric acid ornitric acid to form an uneven surface (“anchor”) for anchoring theconductive layer. The conductor can then be formed by a method combiningelectroless plating and electroplating.

The multi-layer laminate however formed can serve as the startingmaterial in the production of a printed circuit board. Printed circuitboards are prepared by the application of photoresist material to themetallic surface. In the case of a positive photoresist, the pattern ofa circuit is photoimaged onto the surface of the photoresist, therebyphotopolymerizing the positive photoresist. Photoimaging can involvecoherent and/or non-coherent light sources. One method for photoimagingis through a phase mask. The pattern of photopolymerization replicatesthe circuit to be formed. Following photoimaging, the resin-coated foilis subject to a solvent that dissolves the unpolymerized photopolymer.The thus treated resin-coated foil is then subject to chemical treatmentof the exposed metal. After the exposed metal has been removedchemically, the resin-coated foil is then subject to etching, such asion-beam etching, to remove the layer of photopolymerized material,thereby exposing the pattern of conductive pathways beneath, making aprinted circuit board that is now ready to receive electroniccomponents. The printed circuit board so formed can serve as the corelayer of a multi-layer printed circuit board as described supra, or itcan remain a single layer printed circuit board. Depending upon thethickness of the coating prepared from the coating composition, and thenature of the cured or imidized polymer thereof, the printed circuitboard can be rigid or flexible.

In a further embodiment, the cured or imidized layer can be bored with adrill, a laser or the like to form via holes or through-holes.

EXAMPLES Measurements:

The glass transition temperature (T_(g)) and the in-plane thermalexpansion coefficient (CTE) of the cured film was determined using aThermal Mechanical Analyzer by following IPC test method Number2.4.24.5. CTE was determined in a single direction in the plane of thefilm. The glass transition, modulus and loss modulus of the film wasdetermined using a Dynamic Mechanical Analyzer by following IPC testmethod Number 2.4.24.4.

The heat conductivity in a direction normal to the plane of the film wascalculated using this relationship:

Conductivity=Diffusivity×C _(p)×Bulk density.

The specific heat (C_(p)) of the film was measured using differentialscanning calorimetry (DSC) against a sapphire standard using standardmethods. Values of thickness t and bulk density ρ (w/{πr²·t}) are basedon room temperature measurements.

The thermal diffusivity was measured at 25° C. using a Netzsch LFA 447NanoFlash® Xenon Flash Apparatus, available from NETZSCH-Gerätebau GmbH,Selb, Germany. Each 1 inch diameter (25.4 mm) die-cut specimen wassputter-coated with a layer of gold on both sides, followed by sprayingon a layer of graphite. The specimen so prepared was mounted in a holdercooled by a circulating bath to maintain specimen temperature constantduring the measurement. Flashlamp gain was set at 1260 in a range from78 to 5000. Pulse widths of 10 and 66 ms were employed. The detector wasa liquid nitrogen cooled IR detector. Each data point represents theaverage of four specimens.

Example 1 and Comparative Example A

8.0 g of para-phenylene diamine was dissolved in 100 ml of deionizedwater and 57 ml of 0.5M hydrochloric acid at room temperature. 5.1 g ofsodium nitrite dissolved in 50 ml of deionized water was added to thesolution to make the corresponding diazonium chloride. The diazoniumsalt solution was added to a dispersion containing 25 g of hBN (NX1grade, Momentive Performance Materials, Strongsville, USA) dispersed in200 ml of methanol and 800 ml of water. 6.0 g gram of iron powder (325mesh) was added to this dispersion and stirred at room temperature.After five minutes, 250 ml of 0.5 M hydrochloric acid was added to thisdispersion and stirred for an additional 30 minutes (min). Thedispersion was filtered, washed with water, ammonia solution (25 cc ofammonia solution in one liter of water) and then with methanol. Theproduct was dried at 100° C. overnight in a vacuum oven to produce drySMhBN.

Thermogravimetric analysis (TGA—10° C./min heating rate, air atmosphere)showed that the specimen so prepared contained 2.55 weight-% (wt-%)) ofaminophenyl groups on the basis of the weight of the SMhBN based onweight loss between 250° C. to 600° C. Time of Flight-Secondary Ion MassSpectroscopy (ToF-SIMS) analysis of the SMhBN showed positive ion peaksat m/z 92 and 106. The positive ion peak at m/z at 92 confirmed thepresence of —C₆H₄—NH₂groups on the surface of the hBN. The m/z ion peakat 106 corresponded to the structure —N—C6H4-NH2, confirming that theaminophenyl group was attached to the N atom in the hBN through acovalent bond.

FIG. 2 a is a transmission electron micrograph (TEM) of the plateletedges of hBN as received (Comparative Example A). FIG. 2 b is a TEM ofthe platelet edges of the SMhBN prepared according to Example 1. Thevirtually identical appearance of the two samples indicated little or nointercalation of the aminophenyl radical between the platelets of hBN.FIGS. 1 a and 1 b show a ‘d’ (0002) spacing of 3.55 Å and 3.46 Årespectively.

FIG. 3 a. is a TEM of the basal plane in the SMhBN of Example 1, a viewfrom the platelet edges. FIG. 3 b is a TEM of the platelet edges in theSMhBN of Example 1, a view from the basal plane. FIGS. 3 a and 3 bshowed an amorphous coating on the basal plane and none on the edges,thus showing that the surface modification took place only on the basalplane.

Examples 2-5

The procedures of Example 1 were followed but the amount ofpara-phenylene diamine (PPD) was varied. The amounts of sodium nitrite,the hydrochloric acid and the iron were changed proportionately withpara-phenylene diamine on a molar basis. The weight loss data measuredfrom TGA (between 250-600° C.) was taken as the amount of aminophenylgrafting on the hBN surface. Results are shown in Table 2

TABLE 2 Reactant Amount Amount of aminophenyl PPD grafting on the hBNsurface Example (g) Na(NO₂) (g) Fe (g) (wt. %) 2 1.0 0.638 0.75 0.38 32.0 1.275 1.5 1.02 4 4.0 2.55 3.0 1.93 5 8.0 5.10 6.0 2.55

Examples 6-8

The procedure of Example 1 was used to surface modify different gradesof hBN. 4.0 g of PPD was dissolved in 50 ml of deionized water and 28.5ml of 0.5 M hydrochloric acid at room temperature. 2.55 g of sodiumnitrite dissolved in 50 ml of deionized water was added to this solutionto make the corresponding diazonium chloride. This diazonium saltsolution was added to dispersions each containing 25 g of a grade ofhexagonal boron nitride (NX1, PT120 & PT620, Momentive PerformanceMaterials, Strongsville, USA) in 100 ml of methanol and 400 ml of water.3.0 g gram of iron powder (325 mesh) was added to each dispersion andstirred at room temperature. After five minutes, 125 ml of 0.5 Mhydrochloric acid was added to each dispersion and stirred foradditional 30 min. Each dispersion was filtered, washed with water,ammonia solution consisting of 25 cc of ammonium hydroxide in one literof water, then with methanol and dried at 100° C. in a vacuum oven. Theamount of surface functional group was determined by thermogravimetricanalysis. Results are shown in Table 3

TABLE 3 particle surface Amount of hBN size* area* surface graftedExample grade (μm) (m²/g) (%) 6 NX1 0.7 20.0 1.93 7 PT120 12-13 2.0 0.428 PT620 16.0  4.4 0.52 *supplier data

Example 9

This same procedure reported in Example 1 was also used to makehydroxyphenyl grafting on an hBN surface. The PPD was replaced with thesame amount of para-amino phenol. After reaction, TGA showed that theSMhBN thus prepared contained about 0.51% by weight of hydroxy phenylgroups on the basis of the total weight of SMhBN. TOF-SIMS analysis ofthe SMhBN, showed positive ion peaks at m/z 93 and 107. The positive ionpeak at 93 confirmed —C₆H₄—OH groups on the surface of the hBN. The peakat 107 corresponded to —N—C6H4-OH, confirming that the hydroxyphenylgroup was attached to the N atom in the hBN through a covalent bond.

Example 10 and Comparative Example B

A polyamic acid solution was formed, by reacting 1.0 mole of1,3-(4-aminophenoxy)benzene (134APB) with 0.8 mole of4,4′-oxy-3,4,3′,4′-diphthalic anhydride (ODPA) and 0.2 mole ofpyromellitic dianhydride (PMDA) in dimethyl acetamide (DMAc) solvent atroom temperature under a nitrogen atmosphere. The viscosity of thepolyamic acid solution so prepared was 8500 cp at 1.40 s−1 shear rate;the solids content was 19.5% by weight.

Comparative Example B

40.0 g of the above polyamic acid solution was mixed with 7.8 g of theas received hBN and the dispersion was stirred at room temperature for30 minutes. The viscosity of the dispersion was measured using aBrookfield Model DV-II+ Viscometer. Results are shown in Table 4. Thiscomposition corresponded to that of a cured film containing 50% resinand 50% hBN by weight.

Example 10

A similar dispersion to that of Comparative Example B was made using thesame amounts of the polyamic acid, the SMhBN of Example 1, and theviscosity was measured. The data is summarized in Table 4.

TABLE 4 Viscosity (cp) Comparative Shear rate (1/s) Example B Example 101.40 36000 28700 1.12 42120 30000 0.70 52200 32000

Example 11

30 g of SMhBN prepared as in Example 1 was combined with 60 g of DMAcusing a mechanical stirrer to form a dispersion. 51.3 g of the polyamicacid solution of Example 10 was added to the SMhBN dispersion and mixedthoroughly using a Silverson Homogenizer for four minutes at 75% speedto form a SMhBN/polyamic acid concentrate or masterbatch.

56.51 g of the thus made SMhBN/polyamic acid masterbatch was mixed withan additional 29.84 g of the polyamic acid solution using a mechanicalstirrer for about 20 min to make a SMhBN/polyamic aciddispersion/solution that corresponded to a concentration of 55% byweight of SMhBN in the cured film. 25 mg aliquots of ODPA were added tothe SMhBN/polyamic acid dispersion/solution. After each addition, thedispersion/solution was mixed for about ten minutes until the solidsdissolved completely. After a total of six aliquots of ODPA, theviscosity of the dispersion/solution had increased to 77400 cp at 0.70s⁻¹ shear rate. The thus prepared dispersion/solution was kept in adesiccator under vacuum for 30 minutes to remove the trapped air bubblesThe resulting dispersion/solution was cast onto the surface of apolyester film using a doctor blade with a 0.010 inch (in) opening tomake a film 12″ in length and 8″ in width. The two layer film thusformed was baked at 80° C. for thirty minutes in an air circulatingoven. The two layers were separated, and the SMhBN/polyamic acid filmwas clamped into place in a metal frame and baked at 100° C. in an aircirculating oven for fifteen minutes. The B-stage film so produced wasthen cured in a furnace by heating at 5° C./min to 300° C., holding at300° C. for five minutes; then rapidly heated to 375° C. and held at375° C. for five minutes. At the end of the five minute soak, thefurnace was turned off and the thus treated specimen was allowed tocool, still in the furnace, to room temperature.

Comparative Example C

The hBN concentrate was made by mixing 60 g of DMAc and 30 g of theas-received hBN (NX1 grade) using a mechanical stirrer. Then 51.3 g ofthe polyamic acid solution described in Example 10 was added to thisdispersion and mixed thoroughly using a high shear Silverson Homogenizerfor four minutes at 75% speed. 56.51 g of the thus made hBN concentratewas mixed with 29.84 g of the polyamic acid solution using a mechanicalstirrer for about 20 min to make a dispersion/solution of polyamic acidcontaining 55% by weight of hBN in the cured film. ODPA in 25 milligramaliquots was added to the so-prepared dispersion/solution, and mixed forabout ten minutes for each aliquot, until the solids dissolvedcompletely in the dispersion. A total of two such aliquots of ODPA wereadded to the dispersion/solution such that the viscosity of thedispersion/solution increased to 79200 cp at 0.70 s−1 shear rate. Theresulting dispersion/solution was de-gassed, cast into a film, andpost-treated in the manner described for the dispersion/solution ofExample 11.

A control polyimide film was made using the polyamic acid solution ofExample 10. 100 g of the polyamic acid solution was mixed with 100 mg ofODPA using a mechanical stirrer for about 10 min until all the solidsdissolved. The viscosity of the thus prepared polymer solution was 72200cp at 0.70 s−1. The same procedure reported in Example 11 was used tomake and cure a film.

Thermal and mechanical properties of the films produced in Example 11,Comparative Example C, and the unfilled polyimide film were determinedusing both Thermal Mechanical Analysis and Dynamic Mechanical Analysis.The results are shown in Table 5

TABLE 5 Comparison of Polyimide Films Polyimide Film Example 11 Comp.Ex. C T_(g) (° C.)* 244 244 244 CTE (ppm/° C.): 25-200° C. 62 24 31250-290° C. 2806 112 215 T_(g) (° C.)** 236 241 236 Modulus at 25° C.(GPa) 3.522 10.63 11.16 Modulus at 500° C. Film broke at 1.97 0.87 415°C. *Determined by TMA. **Determined by DMA.

Examples 12-14. and Comparative Example D

The SMhBN of Example 1 was combined with polyamic acid solution andconverted to polyimide films as described in Example 1, but in differentrelative proportions, as indicated in Table 6. Table 6 shows the CTE ofthe films so-prespared.

TABLE 6 Film Composition CTE (ppm/° C.) SMhBN(%) Polyimide (%) 50-225°C. 260-300° C. Comp. Ex. D — 100 62 2806 Ex. 12 40 60 34 141 Ex. 11 5545 24 112 Ex. 13 60 40 18 69 Ex. 14 65 35 14 52

Examples 15-22 and Comparative Example D

The SMhBN samples prepared in Examples 6-8 were combined over a range ofproportions with polyamic acid following the procedures in Example 11 toproduce the SMhBN/polyimide composite films listed in Table 7. Thermalconductivity normal to the plane of the films was determined and theresults also given in the table 7.

TABLE 7 Film composition Aminophenyl grafted Polyimide Thermalconductivity hBN (wt. %) (wt. %) (W/m · ° K) Comp. Ex. D 0 100 0.209 Ex.15 55 NX1 45 1.130 Ex. 16 60 NX1 40 1.128 Ex. 17 65 NX1 35 1.356 Ex. 1850 PT620 50 0.993 Ex. 19 25 NX1 + 25 PT120 50 1.033 Ex. 20 30 NX1 + 30PT120 40 1.292 Ex. 21 25 PT120 + 25 PT620 50 1.377 Ex. 22 30 PT120 + 30PT620 40 2.219

Example 23

24.8 g of ODPA,was combined with 4.25 g PMDA to form a mixture. Themixture so formed was added slowly over a period of two hours to asolution of 29.2 g of 1,3-(4-aminophenoxy)benzene (134APB) in 350 mlDMAc to form a 15% polyamic acid solution. After a viscosity of 10000 cpwas reached, 11 g of the SMhBN of Example 1, was added and stirred wellto get a fine dispersion. Then, the viscosity of the polymer was raisedto 75000 centipoise by slowly adding a small amount of PMDA withcontinuous stirring.

The solution mixture was then degassed under vacuum and cast as a filmon a glass plate to make about 12×12″ size film. The thus coated platewas dried for 15 min on a hot plate set at 100° C. The film was removedfrom the hot plate and cooled to room temperature. The film was mountedon a pin frame. The thus mounted film was dried and cured in an ovenprogrammed to increase in temperature from 120 to 300° C. over an hour.Then the frame-mounted film was removed from the oven and immediatelyplaced in a second oven that had been preheated to 400° C. Theframe-mounted film was heated in the second oven for five minutes, andthen removed, and cooled on the bench.

The cooled film was removed from the frame, and the edge trimmed to getan 8×10 in specimen. The so-trimmed film was then placed between two12×12 in (18 um thick) copper foils. The package so formed was theninserted into a larger sandwich consisting of a caulk plate/aluminumfoil, Teflon® PFA sheet/copper foil/SMhBN-polyimide compositefilm/copper foil/Teflon® PFA sheet/aluminum foil/caulk plate. The multilayer sandwich so formed was placed between the platens of a hydraulicpress set at a temperature of 150° C. within a vacuum oven. The presswas then heated to 350° C. and a pressure of 350 psi was applied to theplatens. The press was cooled to 150° C. and the package was removedfrom the press and the laminate removed.

Example 24 and Comparative Examples E and F

An epoxy resin composition was prepared by dissolving in 25 ml of methylethyl ketone 9.0 g of poly(o-cresyl glycidyl ether)-co-formaldehyde,(molecular weight 1080, Aldrich, USA), 0.9 g of PKHH phenoxy resin(Phenoxy Specialties, Rock Hill, S.C., USA), 0.9 g of bishenol A, 3.6 gof Durite® D_SD-1819, a dicyclopentadiene-phenol adduct (Borden ChemicalInc., Louisville, Ky.) and 0.86 g of 2-ethyl-4-methyl imidazole. Thethus formed epoxy solution was stirred using a magnetic stirrer at roomtemperature for 30 min.

14.4 g of the hydroxyphenyl grafted SMhBN of Example 9 was added to theabove-prepared epoxy solution and the resulting mixture was stirred atroom temperature followed by agitation using an ultrasonic probe (DukaneSonic Welder equipped with a lwatt head and ¼″ sonic horn) for fourminutes. The dispersion so produced was degassed under vacuum. The thussonicated dispersion was cast onto a polyester film using a doctor bladeset with a 0.010 in. opening. The cast film was dried in an aircirculating oven at 50° C. for thirty minutes, peeled off the film fromthe base polyester film. The dried film was placed on a porousfiberglass/Teflon sheet, cured in an air circulating oven by heating at130° C. for thirty minutes, at 150° C. for ten minutes and at 170° C.for one hour.

Comparative Example E

A film identical to that of Example 24 was prepared except that the NX1hBN was used as received.

Comparative Example F

And additional film was prepared in the same manner, but without anyhBN.

Table 8 shows CTE results of the three films.

CTE (ppm/° C.) Comp. Ex. F Ex. 16 Comp. Ex. E  25-150° C. 79 27 28170-240° C. 156 56 66

1. A composition comprising hexagonal boron nitride particles having asurface and a substituted phenyl radical bonded to the surface, thesubstituted phenyl radical being represented by the structure:

wherein X is a radical selected from NH₂—, HO—, R²OC(O)—, R²C(O)O—,HSO₃—, NH₂CO—, halogens, alkyl and substituted or unsubstituted aryl; R¹is hydrogen, alkyl or alkoxy, and R² is hydrogen, alkyl or substitutedor unsubstituted aryl.
 2. The composition of claim 1 wherein R¹ ishydrogen, and X is NH₂—.
 3. The composition of claim 1 wherein R¹ ishydrogen, and X is. HO—.
 4. The composition of claim 1 furthercomprising a concentration of 0.1 to 4.0% by weight of substitutedphenyl radical bonded to the surface based on the total weight of thecomposition.
 5. A process comprising reacting particles of hexagonalboron nitride (hBN) with a substituted phenyl diazonium chloride in analcohol/water solution in the presence of metallic iron and HCl to forma reaction product, and recovering the reaction product; wherein thealcohol/water solution has a water concentration of at least 50% byvolume; and wherein the substituted phenyl diazonium chloride isrepresented by the formula

wherein X is a radical selected from NH₂—, HO—, R²OC(O)—, HSO₃—, NH₂CO—,halogens, alkyl, and substituted or unsubstituted aryl; R¹ is alkyl oralkoxy, and R² is hydrogen, alkyl, or substituted or unsubstituted aryl.6. The process of claim 5 wherein R¹ is hydrogen, and X is NH₂—.
 7. Theprocess of claim 5 wherein R¹ is hydrogen, and X is. HO—.
 8. The processof claim 5 wherein the water concentration is at least 80% by volume. 9.The process of claim 5 wherein the molar ratio of substituted phenyldiazonium chloride to hBN is in the range of 0.005:1 to 0.1:1.
 10. Theprocess of claim 5 wherein the iron is added in molar excess withrespect to the substituted phenyl diazonium chloride.